High Temperature Amorphous Composition Based on Aluminum Phosphate

ABSTRACT

A composition providing thermal, corrosion, and oxidation protection at high temperatures is based on a synthetic aluminum phosphate, in which the molar content of aluminum is greater than phosphorus. The composition is annealed and is metastable at temperatures up to 1400° C.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority benefit fromapplication Ser. No. 10/362,869 filed Jul. 15, 2003, which in turnclaims priority from U.S. national application filed under 35 U.S.C. §371 of international application no. PCT/US01/41790 filed on Aug. 20,2001, which is a continuation-in-part of and claims priority benefitfrom application Ser. No. 09/644,495 filed Aug. 23, 2000, now issued asU.S. Pat. No. 6,461,415, each of which is incorporated herein byreference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

The United States government has certain rights to this inventionpursuant to Air Force Office of Scientific Research contract no.F49620-00-C-0022 and Department of Energy contract no. DOEDE-AC05-84OR21400-1DX-SY067V, both with Applied Thin Films, Inc.

BACKGROUND OF THE INVENTION

This invention relates to synthetic inorganic compositions which remainmetastable and possess other desired properties at mid and hightemperature, for example, from 800° C. to 1400° C. and greater.

It is known to use metal oxide coatings for high temperature protectionof substrates or other surfaces. Up to the present time, however, thereare no known synthetic oxides which can remain amorphous and metastableat temperatures up to 1400° C. or greater. Silica, for example, is knownto devitrify/crystallize at temperatures slightly greater than 850° C.Other non-oxide materials, such as silicon oxy-carbide and siliconoxy-nitride rapidly oxidize and form crystalline phases at hightemperatures in air.

Aluminum phosphate is a well known inorganic material that has foundmany uses in applications including catalysts, refractories, composites,phosphate bonded ceramics, and many others. Aluminum phosphate has a lowdensity (d=2.56 g/cm³). It is chemically inert and stable at hightemperatures, as well as being chemically compatible with many metalsand with most widely used ceramic materials including silicon carbide,alumina, and silica over a moderate range of temperatures.

Aluminum phosphate, however, is unsuitable for use as a high temperatureceramic material because it undergoes polymorphic transformations(quartz-type, tridymite and cristobalite) with corresponding large molarvolume changes. Thus, it would be desirable to provide a synthetic formof aluminum phosphate which is metastable and remains substantiallyamorphous at increasing temperatures, or during heating and coolingcycles. Another desirable property would be to provide an aluminumphosphate composition having a low oxygen diffusivity at hightemperatures or in harsh environments, in order to provide oxidationprotection and corrosion resistance to substrates such as metals andceramics.

SUMMARY OF THE INVENTION

The present invention relates generally to substantially amorphousaluminum phosphate materials and/or compositions exhibitingmetastability and various other related properties under hightemperature conditions. In part, metastability can be evidenced byretention of amorphous characteristics under oxidizing conditions, suchmorphology and the extent thereof due at least in part to the aluminumcontent of such materials/compositions, with those having an overallstoichiometric excess of aluminum exhibiting enhanced amorphouscharacter and associated stability as compared to their stoichiometriccounterparts. Such properties and related high-temperature attributescan be effected in the preparation of such materials/compositions,primarily by admixture of the aluminum precursor to the correspondingphosphate precursor to initiate various structural and/or compositionalmodifications which manifest themselves in the high temperatureperformance of the resulting aluminum phosphate material/composition. Inparticular, and as illustrated more clearly in the following examples,aluminum can be identified upon addition to a phosphate precursor andshown as contributing to the amorphous character and associatedmetastability of the resulting aluminum phosphate material/composition.Addition of a stoichiometric excess of aluminum precursor enhances theresulting amorphous character and metastability.

In part, the present invention is a metastable material including analuminum phosphate composition, such a composition as can be representedhaving the formula Al_(1+x)PO_(4+3x/2) wherein x is about 0 to about1.5. The composition of such materials can be characterized bystructural/compositional components absorbing radiation in the infra redspectrum at about 795 cm⁻¹ to about 850 cm⁻¹, and can be furthercharacterized by their presence at temperatures of at least about 1000°C. Without regard to any particular material or compositional phase, invarious preferred embodiments of this invention, as discussed more fullybelow, x is 0 or about 0. In various other preferred embodiments,depending upon desired performance properties of the metastable materialand/or end use application, x is about 0.1 to about 1.0. Generally, suchmaterials are substantially amorphous, the degree to which in partdependent upon the value of x and the aluminum content of the entirecomposition. Depending upon such content and morphology, such materialsare metastable at temperatures at least about 1200° C. As illustratedbelow, in the following examples, such materials can also includecrystalline particles including but not limited to CaWO₄, Al₂O₃ andErPO₄, such inclusions as can result from temperature treatment and/orincorporation of suitable precursor components. Such inclusions can beprovided, as desired, to affect various material physicalcharacteristics and/or performance properties, including, but notlimited to, modification of the material thermal expansion coefficientfor a particular end use or composite fabrication.

In light of the above and in conjunction with the following examples anddetailed descriptions, the present invention can also be a method ofusing the aluminum content of an aluminum phosphate composition toaffect and/or control the metastability thereof. Such a method includesproviding an aluminum phosphate composition from or using an aluminumsalt precursor compound. The resulting aluminum phosphate compositionhas an aluminum content corresponding to that of the precursor compound,such aluminum content sufficient to provide a desired and/orpredetermined compositional metastability. As illustrated below, and aswould be understood to those skilled in the art, metastability of such amaterial can be evidenced spectroscopically showing an amorphous and/ornon-crystalline aspect of the material. The material can have a certainmetastability with an aluminum content stoichoimetric with respect tophosphate. Generally, the metastability of such a material can beenhanced with a stoichoimetric excess of aluminum. Aluminum content andresulting stability can be effected upon preparation of thecorresponding precursor and admixture with a suitable phosphateprecursor, as illustrated elsewhere herein. Metastability and variousother optical, chemical and/or physical properties can benefit byinclusion of other metal components including but not limited tosilicon, lanthanum and zirconium upon choice of and/or modification of asuitable precursor.

Accordingly, the present invention also includes, in part, a method forpreparing a precursor solution for the formation of a metal phosphatecomposition, preferably an aluminum phosphate composition. Such a methodincludes preparation of a first solution of a metal and/or aluminumsalt; preparing a phosphorus component; and admixing the solution andcomponent. Typically, the phosphorus component is an alcoholic solutionof phosphorus pentoxide, but other phosphorus components/phosphateprecursors can be used with comparable effect as described elsewhereherein. Likewise and without limitation, the metal/aluminum component isprovided in alcoholic solution, with choice of solvent dependent uponmetal/aluminum solubility and compatibility with the correspondingphosphorus/phosphate component. Among the various departures from theprior art, this aspect of the present invention contemplates use of astoichiometric excess of the corresponding metal and/or aluminumcomponent in the preparation of such a precursor and use thereof in thesubsequent formation of the desired metal and/or aluminum phosphatematerial/composition. As described more fully elsewhere herein,preferred embodiments of this invention are directed toward suitablealuminum salt components, precursors and resultingmaterials/compositions, but various other metal components can beincorporated into the precursor solution to provide the resultingmaterial/composition associated thermal, optical, chemical and/orphysical properties.

In part, the present invention also includes a method of using analuminum phosphate composition to enhance the oxidation resistance of anassociated substrate. The method includes (1) providing an aluminumphosphate composition of this invention, preferably one having theformula A1 _(1+x)PO_(4+3x/2), wherein x is about 0 to about 1.5; and (2)applying the composition to a suitable substrate. In various preferredembodiments, depending upon end use application and/or fabricationtechnique, the composition can be annealed either prior or subsequent tosubstrate application. Regardless, as illustrated below, such acomposition can be dip-coated to provide a film on the substrate.Alternatively, without limitation, the composition can be prepared as apowder then either plasma—or aerosol—sprayed onto a substrate.

In part, the present invention can also include an aluminum phosphateproduct with aluminum-oxygen-aluminum structural moieties, absorbingradiation in the infra red spectrum at about 795 cm⁻¹ to about 850 cm⁻¹.Such a product is obtainable and/or can be produced by (1) mixing analcoholic solution of phosphorus pentoxide with a solution of analuminum salt, the salt either stoichiometric or in stoichiometricexcess with respect to the phosphorus precursor; and (2) heating theresulting admixture. Such a product is substantially amorphous, but canalso provide for crystalline particulate inclusions, as discussed above.As a representative embodiment, such particles are crystalline erbiumphosphate inclusions prepared by incorporating an erbium salt with theaforementioned aluminum salt solution. Alternatively, illustratinganother aspect of this invention, the product can include Group III Aand/or Group III B-VI B metal oxide particles in amounts sufficient tomodify the thermal expansion coefficient of the resulting product.

With regard to one or more aspects of the preceding discussion, thepresent invention can include a new class of phosphate compoundsformulated to contain an excess amount of metal species in thecomposition; that is, with reference to a preferred embodiment, thealuminum atoms exceed the number of phosphorus atoms found instoichiometric aluminum phosphate. The excess can be more than onepercent and preferably greater than five percent.

Whether compositions of this invention are stoichiometric or reflect anexcess metal component methods for their preparation include thosedisclosed in U.S. Pat. No. 6,036,762, the entirety of which isincorporated herein by reference. In accordance therewith, a precursorsolution is formed from two liquid components. The first component is ametal salt dissolved in alcohol. The second component of phosphoruspentoxide is dissolved in alcohol. The two components are then mixedtogether in the desired molar proportions to provide a stable precursorsolution, with the phosphate portion at least partially esterfying toform a polymer-like structure which uniformly entraps the metal ion.

The solution, as such, may be heated directly to remove the alcoholportion and other species and provide a pure metal phosphate.Preferably, however, the solution is applied as a coating to anon-porous or porous substrate using any suitable method, and the coatedsubstrate is heated, typically to a temperature of less than 600° C., toobtain a uniform and pure coating of the metal phosphate on thesubstrate.

A particular advantage of this approach is that the precursor solutionprovides a substrate coating of even and uniform thickness for substrateapplication. After initial heat treatment, subsequent coatings may beapplied to increase coating thickness. This methodology is applicable tothe formation of precursor phosphate solutions containing more than onemetal ion. The ability to adjust the concentration of the compositesolution over a wide range is another distinct advantage, allowing forprecise or controlled amounts of metal phosphates to be formed.

Further and as directed more particularly to the present invention, theaforementioned admixture/precursor solution can be dried and thenannealed, for example, at temperatures of up to 800° C. or greater, inair. The annealing step is believed to cause a transformation of themolecular structure, with the final product being more than 50%amorphous in content, and with the amorphous nature being sustained forlong periods at temperatures up to 1400° C. or greater withoutoxidation. Depending on the synthetic procedure and presence of othercomponents or additives, the composition may also contain smallcrystalline inclusions which can impact other desirable properties, suchas toughness and optical activity. The composition exhibits otherdesirable properties, such as very low oxygen diffusivity, low thermalconductivity and high emissivity. Thus, a particularly suitableapplication is to use the composition as a coating on a substrate tominimize oxidation of the substrate at high temperatures.

The initially formed organic solution can be converted into any desiredform. For example, the solution may be applied to a metal, ceramic orother substrate, such as ceramic composites and then annealed, or it maybe converted into any desired shape, such as fibers or filaments or inany other desired molded form, or may be converted into a powder forapplication to substrates using a suitable spray technique. Variousother end use applications are provided elsewhere, herein. Variousmaterials/composites of this invention are available under the Cerablaktrademark from Applied Thin Films, Inc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Uncoated and AlPO₄ coated type 304 stainless steel after 1000°C. 100 h. The coated piece showed remarkably little weight gain fromoxidation (0.08-0.24%) compared to the uncoated piece (4.5-8.6%).

FIG. 2. TEM micrographs of powders annealed to a) 1200° C. 420 h b)1300° C. 100 h c) 1400° C. 10 h d) electron diffraction 1200° C. 100 h.

FIG. 3. TEM micrograph of a coating on a Nextel 720 fiber, with analumina overcoat, annealed 1200° C. 2 h.

FIG. 4. XRD pattern of a stoichiometric material annealed to a) 1100° C.for 1 hour, b) 1100° C. for 163 hours. Note the splitting of the peak at21.5, indicating the presence of crystalline tridymite and cristobalitephases.

FIG. 5. a) XRD pattern of an aluminum phosphate material (x=0.75)annealed at 1100° C. for 1 hour, b) XRD pattern of the same compositionannealed to 1100° C. for 163 hours. Note the lack of differentiation ofthe tridymite peaks.

FIG. 6. Thermal expansion measurements for an aluminum phosphatecomposition, in accordance with this invention.

FIG. 7. TEM micrograph and electron diffraction pattern of AlPO₄ coatedNextel 720 fiber annealed to 1200° C. for 100 hours.

FIG. 8. TEM bright field image of AlPO₄ nanocrystal embedded in theamorphous matrix.

FIG. 9. TEM micrograph of Er-doped aluminum phosphate annealed to 1000°C. for 1 hour.

FIG. 10. a) Thermal conductivity of AlPO₄ (lower line) with YSZ, fusedsilica, mullite alumina and spinel. b) Thermal conductivity of AlPO₄(lower line) and YSZ, a common thermal barrier material.

FIG. 11. SEM micrograph of a cross-section of an AlPO₄ fiber.

FIG. 12. X-ray diffraction patterns of a) powder annealed at 1100° C., 1hr and b) powder annealed at 1200° C., 500 hr, 10 atm, 15% steam(white).

FIG. 13. TEM micrograph of crushed white pellet of Example 31, showingnanocrystalline inclusions in an amorphous matrix and electrondiffraction pattern from different area of the same sample.

FIG. 14. Raman spectra from AlPO₄ annealed 1400° C., 1 hr; a) blackarea, and b) white area from the same material/composition sample.

FIG. 15. ³¹P NMR of phosphorus pentoxide dissolved in ethanol. a) soonafter dissolution b) after 24 hours reflux.

FIG. 16. Liquid ³¹P NMR spectra of admixed precursor solutions, showingthe effect of aluminum addition. a) full spectrum b) plot showing peaksreflecting the presence of aluminum.

FIG. 17. X-ray diffraction patterns of the solutions shown in FIG. 16after anneal to 1100° C., 160 hrs. a) full scale b) zoom to show thedifferences in the peaks near 21°.

FIG. 18. FTIR of stoichiometric and non-stoichiometric compositionsannealed to 1100° C. for 1 hr.

FIG. 19. Deconvolution of ²⁷Al NMR spectrum of stoichiometric AlPO₄annealed to 1100° C., 1 hr.

FIG. 20. Deconvolution of ²⁷Al MAS NMR of an excess aluminum compositionAl/P=2 (1-fold excess, x=1.0) after anneal at 1100° C., 160 h.

DETAILED DESCRIPTION OF SEVERAL PREFERRED EMBODIMENTS

As discussed above, the present invention relates to a new class ofmetastable high-temperature amorphous inorganic compositions. A uniqueamorphous structure can be derived using a simple, low-cost sol-gelprecursor. The thermal stability of the amorphous material is primarilycontrolled by metal content of the corresponding precursor, aluminum inpreferred embodiments. Several compositions have been synthesized inamorphous form and shown to be stable for hundreds of hours above 1200°C. Most crystalline materials of the prior art synthesized using sol-gelroutes undergo amorphous to crystalline transition below 1000° C. Inthis case, however, and with aluminum phosphate as a generic example,thermodynamic equilibration to stable crystalline alumina and AlPO₄phases does not occur until annealing above 1500° C. Calorimetricmeasurements revealed highly exothermic dissolution behavior suggestingthe material to be thermodynamically unstable or metastable. Extremelylow oxygen diffusivity in the amorphous material, which may beattributed to a special “Al—O—P” cluster, appears to be dominating thesluggish kinetics. Hermetically dense and adherent thin films (1000 Å orless) deposited by a simple dip coating process demonstrate remarkableability of the material to protect stainless steel from oxidation at1000° C. (see, FIG. 1 and Example 2, below).

When prepared as a film or coating, the material tends to remaincompletely amorphous whereas powders derived therefrom contain amorphousmaterial with minor amounts, up to about 20-30%, of nanocrystallineinclusions (varying in size from 5-60 nm) of stoichiometric aluminumphosphate (FIGS. 2 and 3). Likewise, as described elsewhere herein, theamorphous content and presence of nanocrystalline inclusions can also beaffected by the stoichiometry of an aluminum precursor, with the use ofa stoichiometric excess thereof reducing the incidence of suchinclusions, increasing amorphous content and enhancing the metastabilityof the entire material/composition. Several properties characterizingsuch compositions of this invention are provided in Table 1, below.

TABLE I Selected Illustrative Properties Oxygen Diffusivity ~1 × 10⁻¹²cm²/sec (chemical diffusivity @ 1400° C.) Thermal expansion 5 × 10⁻⁶ K⁻¹Thermal conductivity 1.0-1.5 W/mK (RT-1300° C.) Dielectric constant3.3-6.35 (for x = 0.5-0.75)

The very low oxygen diffusivity allows for the use of extremely thinamorphous protective coatings (50-100 nm) where cracking due to thermalstresses is less of a concern. This unique property can be exploited toprovide protection for many metals and ceramics used in high temperatureapplications. Formation of nanocrystalline glass-ceramic composites mayalso provide the opportunity to tailor physical, thermal, mechanical,and optical properties for a number of applications. Thematerials/compositions of this invention can be formed as a continuousfilm or as a powder (that can be plasma sprayed) or in a near-net shapedconsolidated form. Some of the potential applications include oxidationand corrosion protection (low oxygen diffusivity and chemicaldurability), thermal protection for aerospace and space vehicles (highemissivity, low thermal conductivity, and low oxygen diffusivity), lowobservable thermally stable coatings (low dielectric constant),protection against molten metal (non-wetting character), interfacecoatings (non-wetting) and matrices (high strength and ease offabrication) for ceramic matrix composites (CMCs).

A preferred method for making the composition of the present inventionis described in the aforementioned U.S. Pat. No. 6,036,762. An aluminumsalt, such as aluminum nitrate having water of hydration is dissolved inan organic solvent, preferably an alcohol such as ethanol. A quantity ofphosphorus pentoxide (P₂O₅) is dissolved in a separate container in thesame solvent. The molar ratio of Al to P in the Al solution is greaterthan a one-to-one ratio with phosphorus and is preferably at least 1%and most preferably at least 5% greater. The upper practical limit ofexcess aluminum has not been determined, but compositions containing tentimes excess aluminum have been prepared, and a 1.5 to 3.5 excess molarratio appears to be most promising in terms of retaining the amorphouscontent at high temperatures.

More generally, as applicable to broader aspects of this invention, thissynthetic approach provides a metal phosphate precursor solution fromtwo separate liquid components using a common organic solvent. While avariety of organic solvents may be potentially useful, liquid alcoholsare preferred, such as methanol or ethanol, with ethanol being mostpreferred. Accordingly, a first component of the precursor solution isprepared from a metal salt dissolved in alcohol such as ethanol. Amixture of salts of different metals may be employed. Nitrates,chlorides, acetates or any salt of metal soluble in alcoholic media maybe used. The choice of metal salt and/or alcohol is limited only byassociated solubility considerations.

The salt of any metal may be employed in the first component. For thepreparation of coatings for use in high temperature reactiveenvironments, reference is made to U.S. Pat. No. 5,665,463, incorporatedherein by reference. The metal salt may comprise a monazite having thegeneral formula MPO₄, where M is selected from the larger trivalent rareearth elements or the lanthanide series (La, Ce, Pr, Nd, Pm, Sm, Eu, Gdand Th). Xenotimes as described in the above patent can also beprepared. Other di- and tri-valent metals such as aluminum areespecially suitable.

The second component of the precursor solution is phosphorus pentoxide(P₂O₅) dissolved in the same solvent such as ethanol. Withoutlimitation, there is believed a controlled reactivity between thealcohol and phosphorus pentoxide in which phosphate esters are produced.The esterification process continues, forming ester chains while thesolution ages, and the solution becomes sufficiently polymeric such thatgood film forming properties are attained. The metal salt solution ispreferably added to the phosphorus pentoxide solution shortly afterpreparation of the latter and before extensive esterification occurs.

The precursor solution can be prepared at a variety of concentrations,depending on the desired film microstructure. For example, usinglanthanum nitrate, a solution providing up to 160 grams per liter yieldof lanthanum phosphate can be obtained. The metal salt and the phosphateare provided in the mixture in either stoichiometric proportions or withan excess of metal salt to yield the desired metal phosphate.

As described more fully elsewhere, herein, the solution comprising thetwo components is shelf stable and can be converted to metal phosphateby heating. Since the solution has good wetting and coating properties,however, the preferred method of employment is as a coating on porousand non-porous substrates. For example, lanthanum phosphate hassubstantial utility as a coating on ceramic fibers, fabrics or in otherstructures used at high temperatures, i.e., in excess of 1200° C. Thephosphate coating allows for increased toughness for the composite asreferred in U.S. Pat. No. 5,665,463. The solution may be applied as acoating on non-porous materials such as metals and metal alloys.

Upon pyrolysis of the precursor coated substrate, much of the solventevaporates at a relatively low temperature, leaving a continuous film ofresidual precursor material on the substrate. Upon additional heating,all species except for metal and phosphate are removed, leaving acoating of the metal phosphate. The temperature to which heating isrequired may be evaluated by differential thermal analysis. For theLaPO₄ precursor, heating to a temperature of 600° C. for a brief periodassures total conversion. X-ray diffraction of the film obtainedconfirmed the formation of a single phase lanthanum phosphate. Scanningelectron microscopy analysis of the film showed it to be smooth,uniform, continuous and stoichiometric. The use of a volatile solventsystem allows the metal phosphate to form at relatively lowtemperatures.

The precursor liquid can be coated onto a suitable substrate, such as ametal or alloy or ceramic or mixed with particles of ceramic materialrequiring oxidation and/or corrosion protection. In addition, the liquidcan be drawn into fibers, placed in a mold, or used alone. Regardless,the liquid is converted into solid, stable form by annealing orpyrolysis in air. Typically, for aluminum compositions, this requiresheating to temperatures normally above 750° C. for a period of time, forexample, for one hour, or at higher temperatures. Complete annealingbecomes evident when the composition assumes a black or dark grey color.

With regard to at least the aluminum phosphate compositions of thisinvention, it is believed that the decomposition behavior of organicbased precursor at least partially controls the molecular events leadingto a unique inorganic compound. The material contains in excess of 50%of an amorphous compound and may also contain nanocrystals. The materialremains amorphous and metastable when heated to temperatures fromambient and up to 1400° C. or greater for extended period of time. It isbelieved that increased storage time of the precursor solution increasesamorphous content.

Based on initial observations, it has been found that the amorphouscontent of the annealed composition of the present invention may beinfluenced by at least two factors, namely, application and age of theprecursor solution. As an example of the first effect, coatings ofsolution applied on fibrous substrates appear to be substantiallycompletely amorphous even after annealing at 1200° C. for two hours.This has been initially confirmed by TEM analysis of solution coated andannealed on mullite-alumina fibers with an overcoat of alumina. On theother hand, powders synthesized in alumina crucibles at 1000° C. for 30minutes contain a significant fraction of AlPO₄ crystallites. Aging ofthe precursor solution appears to have a significant effect on thephosphorus environment in the precursor as well as the amorphous contentin the pyrolyzed product. Storage of the solution in a refrigerator fora period of up to two years or at room temperature for over one monthtends to yield more pure amorphous content.

Of the AlPO₄ samples tested, the composition/material had a low densityin the order of 1.99 to 2.25 g/cm³, in comparison with 3.96 g/cm³ foralumina. The composition exhibits low oxygen diffusivity; in samplesconducted containing 75% excess aluminum the chemical diffusivity was inthe order of 1×10⁻¹² cm²/sec at 1400° C. The material also exhibits ahigh emissivity, potentially useful in thermal protection systems, suchas space applications. Thermal conductivity has been measured at 1 to1.5 W/m.k. The material is inert in various harsh environments, and hasa non-wetting character to most materials, including molten aluminum andsolid oxides. Coatings as thin as 0.25 microns are capable of protectingmetallic and other surfaces.

Potential applications include thermal, corrosion and oxidationprotection for metals and metal/ceramic-based thermal protectionsystems, high emissivity coatings, interface coatings for siliconcarbide and oxide based ceramic matrix systems, environmental barriercoatings for metal and ceramic based systems, fibers for composites andfiber lasers, corrosion protection in molten metal processing,monolithic materials for thermal insulation, catalyst supports, as wellas many others. The material may also possess a low dielectric constant,making it useful in Radome applications.

Examples of the Invention Example 1

To make 850 mL of 75.46 g/L a precursor solution to synthesize theamorphous aluminum phosphate material with a 1.75:1 Al:P ratio (0.375molar excess Al₂O₃), 408.94 g Al(NO₃)₃9H₂O was dissolved in 382 mlethanol to make 500 ml of solution. In a separate container in an inertatmosphere, 25.23 g P₂O₅ was dissolved in 300 ml ethanol. After the P₂O₅is dissolved, the two solutions were mixed together and allowed to stirfor several minutes. After the solution was thoroughly mixed, it wasplaced in a large container in an oven at 150° C. for one or more hours.After the resulting powder is completely dried, it was annealed in airto 1100° C. for one hour to form amorphous aluminum phosphate powderwith 0.75 moles excess aluminum per mole aluminum phosphate.

Example 2

To form an oxidation resistant amorphous aluminum phosphate coating on arectangular coupon of 304 stainless steel, the piece was dipped in theprecursor solution of Example 1, diluted to a certain concentration andremoved. The sample was dried in flowing air to remove the solvent. Thesample was dried more thoroughly in an oven at 65° C. The piece wasannealed in air to 1000° C. (at a ramp rate of 10 C/minute) for 100hours and cooled to room temperature at 10 C/minute, along with anuncoated piece of 304 stainless steel of the same size and shape. Theweight of each uncoated piece was measured prior to anneal. The weightwas measured again after coating and anneal. The amorphous aluminumphosphate coated piece showed remarkably less weight gain. The weightgain data is given in the table below.

TABLE I Weight gain of uncoated, and coated (AlPO₄, with 75% excess Al)stain- less steel coupons annealed to 1000° C. in air. The weight gainis related to the weight of the annealed, uncoated coupon. OriginalWeight after Weight % Weight Sample weight (g) anneal (g) gained (g)gained Amorphous 20.3727 20.4207 0.048 0.24% aluminum phosphate (incl.coating) Uncoated 20.6303 22.4123 1.782 8.64%

Example 3

To form an amorphous aluminum phosphate coating on a solid substrate byplasma spray, amorphous aluminum phosphate powder made in Example 1 ismilled to a small and uniform size (around 20 microns) in a ball mill.The powder is then deposited using the small particle plasma sprayprocess (see, U.S. Pat. No. 5,744,777 incorporated herein by referencein its entirety).

Example 4

Bulk amorphous aluminum phosphate is formed by electroconsolidation(U.S. Pat. No. 5,348,694). Finely ground amorphous aluminum phosphatepowder was mixed with a binder (1 wt % PEG 8000 and 2 wt % PEG 20M) andthen pressed into a pellet. This pellet was pre-sintered at 1200° C. forfive hours. The pellet was then electroconsolidated at 1300° C. for 30minutes. The final pellet had a density of 1.99 g/cm³.

Example 5

Amorphous aluminum phosphate fibers were made from viscous polymerformed from the precursor solution of Example 1. The AlPO₄ solution wasdried at 50-65° C. until 40-30% of the weight is retained. The residuehad a mainly clear, glassy appearance with a high viscosity. Greenfibers were pulled with a needle, inserted into the viscous residue andquickly removed. The fibers were dried immediately in flowing air at650° F. The green fibers were then annealed to at least 900° C. to formamorphous aluminum phosphate fibers.

Example 6

Rare earth and other metal ions can be incorporated into the amorphousaluminum phosphate structure. An erbium doped precursor solution with0.75 moles excess metal (aluminum and erbium) of which 5 mol % is erbiumwas synthesized in a manner similar to the amorphous aluminum phosphatesolution of Example 1. 31.2 g Al (NO₃)₃9H₂O was dissolved in 75 mlethanol. In an inert atmosphere glove box in a separate container, 1.94g Er(NO₃)₃5H₂O was dissolved in 20 ml ethanol. The erbium nitratesolution was added to the aluminum nitrate solution and left to stir forseveral minutes. In a separate container in an inert atmosphere glovebox, 3.55 g P₂O₅ was dissolved in 40 mL ethanol. After the P₂O₅ wasdissolved, the aluminum nitrate and erbium solution was added and leftto stir for several minutes. The solution was then dried at 150° C. forabout an hour and annealed to 1000° C. for one hour. X-ray diffractionof this material annealed to 1000° C. for one hour confirms theamorphous structure, with no erbium phosphate crystalline.

Example 7

FIG. 4 is a typical x-ray diffraction pattern (XRD) pattern obtainedfrom stoichiometric aluminum phosphate synthesized from an ethanolicprecursor solution containing nominal equimolar amounts of aluminumnitrate nonahydrate and phosphorus pentoxide. The solution was dried andthe powder obtained was calcined to 1100° C. in air for one hour and is“jet” black in color. It is immediately evident from the pattern thatthe material is not fully crystalline and may contain a significantamount of crystalline disorder or amorphous content. Closer examinationof the broad peaks reveals the presence of disordered tridymite andcristobalite forms of AlPO₄. Further annealing of this material forlonger times in air (1100° C., 163 hours) does induce significantcrystallization as seen in FIG. 4 where the tridymite peaks are muchbetter defined, and the cristobalite peak is separate from the maintridymite peak.

In contrast, FIG. 5 shows the XRD pattern of aluminum phosphatesynthesized with excess aluminum (x=0.75, 75% molar excess) in theprecursor solution. The striking difference between the patterns inFIGS. 4 and 5 is immediately apparent: the diffraction pattern for thematerial with excess aluminum retains broad, low-intensity peaksindicative of a large degree of non-crystalline amorphous structure andenhanced metastability.

Without restriction to any one theory or mode of operation, theprecursor design, along with excess aluminum, is believed a factor inthe preparation of the present compositions. The poly-esterification ofP₂O₅ by ethanol and hydrolysis controls the chemistry of clusters inliquid during which time a sequence of molecular events occur yieldingunique spatial coordinations between P, Al, O, and —OH which ispreserved through gelation and calcination. Synthesis of AlPO₄ withexcess aluminum significantly enhances the thermal stability of theresulting material/composition.

Example 8

The addition of excess aluminum to the precursor solution results in thepresence of a substantial number of coordinations other than regulartetrahedral coordinations, including but not limited to distortedoctahedrally coordinated aluminum, in the pyrolyzed product. Crystallinealuminum phosphate of the prior art consists of tetrahedrallycoordinated aluminum and phosphorus, but ²⁷Al MAS NMR of the aluminumphosphate materials/compositions of this invention shows the presence ofboth 4- and 6-fold aluminum (see, also Examples 35a and 35b, and FIGS.19 and 20, below) consistent with the metastability exhibited therewith.

Example 9

Thermal expansion of an electroconsolidated aluminum phosphate pelletwas measured by dilatometry from room temperature to 1100° C. (FIG. 6).The thermal expansion coefficient is considerably lower than steel whichhas a thermal expansion coefficient around 13×10⁻⁶/K. But, as evidentfrom stainless steel coating experiments, very thin coatings of suchmaterials are able to withstand the thermal expansion mismatch andremain adherent and crack free even after heating to 1000° C. andreturning to room temperature.

Example 10

TEM analysis of a 50 nm coating on Nextel 720 alumina/mullite fiberannealed to 1200° C. for 100 hours shows that the aluminum phosphatecomposition of this invention has remained completely amorphous (FIG.7). No nanocrystalline inclusions are evident.

Example 111

Nickel-based superalloys are frequently used in high temperatureapplications such as turbine blades. However, oxidation at high usetemperatures is still a problem. Ni-based superalloy pieces were coatedwith an AlPO₄ material of this invention to substantially reduce thekinetics of alumina scale growth and spallation, demonstrating thisinvention dramatically reduces high-temperature oxidation.

Example 12

Aluminum phosphate powders synthesized as described herein can containnanocrystalline inclusions embedded in the amorphous matrix, in contrastto completely amorphous material obtained as a thin film. TEMexamination of annealed powders showed two distinct types of material.After 1 hour anneal at 1100° C., about 20-30% of the powder samplecontained isolated aluminum phosphate crystallites. Most of the sample,however, contained an amorphous/glassy matrix with nm-sized crystallineinclusions that ranged from 5-30 nm (FIG. 8) and well dispersed. TEMstudies of powder annealed to 1300° C. for 100 hours shows that theoverall fraction of nanocrystals in the material is essentially thesame, with grain size increasing slightly (25-60 nm).

Example 13

Similar results were obtained with ErPO₄ nanocrystallites in a matrix. 5mol % Er-doped powders were prepared as described elsewhere, herein. TEManalysis of Er-doped material annealed to 1000° C. for 1 hour shows anincrease in nanocrystalline fraction over the undoped material (FIG. 9).EDS confirms that Er is present in these nanocrystals. XRD analysis ofEr-doped material annealed to 1100° C. for 1 hour shows definite ErPO₄peaks.

Example 14

The compositions of this invention have a low thermal conductivity(1.0-1.5 W/mK)-lower than yttria-stabilized zirconia, a commonly usedthermal barrier coating material (FIG. 10). Such materials thereforeshow potential as providing both an environmental and thermal barriersimultaneously, and as can be achieved in application of in one coating.Thermal barrier coatings are frequently plasma sprayed, which involvedpartial melting of the powder. An AlPO₄ powder has been plasma sprayedonto steel and cast iron, and the XRD pattern does not indicate anychanges in the structure.

Example 15

An AlPO₄ composition of this example is prepared as smooth, denseamorphous ceramic fibers, having high strength and high creep resistancein the absence of grain boundaries where flaws form easily. A highstrength, creep resistant fiber stable to high temperatures would havegreat structural potential. (See, FIG. 11 and several other examples,below.)

Example 16

The aluminum phosphate materials/compositions of this invention arenon-wetting and non-bonding. With thermal stabilities up to 1400° C.,they may provide a high-temperature substitute for Teflon®-likenon-stick coatings in applications ranging from cookware to enginecomponents.

Example 17

A slurry of fine particles in solution can be applied by aerosolspraying onto a substrate. Accordingly, a slurry of AlPO₄ powder(average particle size=16 microns) is mixed into an AlPO₄ solution in aratio of 5 g powder/100 mL solution. This slurry is aerosol sprayed ontoa heated stainless steel coupon. The result is a coating of AlPO₄particles embedded in an AlPO₄ coating. The coating adheres well to thesurface of the steel.

Example 18

A coating of a composition/material of this invention is obtained bychemical vapor deposition (CVD). CVD coatings can be deposited at lowtemperature, thereby creating an amorphous coating. CVD also allows forgood stoichiometry control. Accordingly, aluminum acetylacetonate andtrimethyl phosphate are dissolved in toluene. This solution is placed ina liquid-delivery assisted CVD system. This liquid precursor will allowfor careful mixing and stoichiometric control. The solution istransferred into a flash evaporator, where it is vaporized. This vaporenters into the reactor, and reacts and deposits as a solid on thesubstrate.

Example 19

Another route for the preparation of compositions of this invention canbe through a reaction of a solid with a liquid phosphorus source(phosphoric acid, phosphorus pentoxide solutions, etc.). The solid maycontain aluminum, which will promote the formation of amorphous aluminumphosphate. Accordingly, a solid containing a small amount of aluminum isdipped in phosphoric acid. When this solid is heated above 800° C., thephosphorus on the surface reacts with the small amount of aluminum toform amorphous aluminum phosphate.

Example 20

Composite coatings of this invention can be deposited on a substrate.Solid particles are added to an AlPO₄ solution to form a slurry. Thiscomposite coating can be deposited on a substrate by dip coating,applying with a brush, aerosol spraying, etc. When this coating is fullyformed under a heat lamp or in a furnace, a composite coating,containing particles embedded in an AlPO₄ coating is produced. Theparticles can be of any composition.

Example 21

Glasses are susceptible to corrosion in a variety of atmospheres, fromdistilled water to humid air. Sodium silicate glass is a common glassand is very susceptible to corrosion whether the glass is immersed inliquid, being rained on, or simply being stored in a humid warehouse.Glass containers are susceptible to corrosion from the liquid they arecontaining. Water and acidic and basic media promote corrosion of glass.Glass windows are subject to corrosion when it rains. Glass that isbeing stored is subject to localized pitting as atmospheric humiditydeposits as drops on the surface.

Through an ion exchange reaction with hydrogen ions, the sodium ionsdissolve into the surrounding water. The hydroxyl content in the waterdissolves the silica as well, but this process is much slower.

Several methods of combating glass corrosion are used. Some commonmethods of increasing durability of commercially produced glassesinclude: the addition of other components to the melt and forming aprotective coating. CaO, Al₂O₃ and MgO are commonly added to the sodiumsilicate melt to retard leaching of the sodium. Surface coatings areprovided by treatment with SO₂ gas to form sodium sulfate and annealingthe glass in a trace fluorine atmosphere.

The compositions/materials of this invention provide a transparentcoating on glass. The coating goes down very smoothly, and the coatingis only readily visible in areas where it has been disturbed (such aswhere it was held during dip-coating). Such coatings could be used toincrease chemical durability without decreasing transparency. Thisinvention could provide a protective layer, which limits the transportof hydrogen or hydroxyls to the glass surface, and the transport of thecorrosion products out of the glass.

Example 22

Aqueous solutions of this invention are desired as a non-flammable,non-toxic alternative to their alcoholic counterparts. Aqueous solutiondoes not require special hazardous labeling during shipping, does notrequire the large amount of ventilation in the workplace, and is moreattractive to manufacturers accustomed to working with aqueous systemsand processes. Dried AlPO₄ gel was produced by heating solution at 100°C. in a convection oven. This dried gel is white and fluffy. This gelwas dissolved into deionized water. The gel goes into solution easily,forming a viscous, yellowish solution. When annealed to 1100° C. for 1hr, the XRD pattern shows the typical aluminum phosphate diffractionpattern. When annealed to 1000° C. for ½ hr, the XRD pattern showed anamorphous hump, that is typical of an aged composition/material. Thepowder is jet black and glassy in appearance.

An aqueous solution can be made much more concentrated: up to 25% byweight AlPO₄ versus 10-15 wt % in ethanol. This solution was coated ontoa glass slide by standard dip coating. A concern with aqueous solutionsis that the film formation characteristics are different and preparationof a continuous, smooth film may be more difficult than through use ofan alcoholic solution.

Example 23

With reference to the preceding discussion regarding precursorsolutions, a viscous, clear liquid can be prepared from which fibers canbe pulled by inserting and retracting a needle. The fiber precursor ismade by concentrating precursor solutions in a rotary evaporator toapproximately 30 wt % concentration. The fiber precursor can bedifficult to prepare at the risk of becoming too concentrated. The fiberprecursor is unstable on its own. After concentration, a clear liquid isleft. This liquid is stable for 10 minutes-hours, but will eventuallyspontaneously decompose in a strong exothermic reaction. The resultingfiber precursor can be used, but there is generally a lot of foam.However, if the solution is put in a water bath immediately afterremoving it from the rotary evaporator, it keeps the decomposition fromproceeding so violently, and a clear, slightly yellow liquid results.Accordingly, 100 mL of 9.1 wt % solution is condensed to 40 mL in arotary evaporator. The temperature was 60° C., and the pressure wasvaried to keep the ethanol evaporating. After the solution wasconcentrated, it was poured into a jar and kept in a water bath to sit.After 15 minutes, the decomposition started, and clear, viscous, yellowliquid remained.

Example 24

Intended fiber applications include a) structural ceramic fibers used inceramic matrix composites, metal matrix composites—currently, SiC and avariety of oxide fibers are being developed, b) fiber-optic amplifiers,and c) fiber lasers. Fibers have been hand-drawn from fiber precursor(for scale-up, the precursor will be fed into a spinerette to producecontinuous single or multi-filaments (typically 10 microns in diameter.The fibers are drawn by putting a thin rod into the precursor, thenquickly withdrawing it. The resulting fibers are smooth and dense. Thediameter is not uniform, but due only to the hand drawing process. Thefibers show stability up to 10 hours at 1200° C., but after 100 hours at1200° C., dramatic phosphorus loss is seen. An attractive advantage ofsuch fibers will be the use of nanocrystalline inclusions within theamorphous matrix to improve its strength, toughness, creep resistance,and thermal expansion properties. Accordingly, a small metal spatula isimmersed slightly in the precursor from the preceding example. Thespatula is withdrawn at a steady rate, and the fiber is caught on apiece of stainless steel mesh. The mesh is bent into a C shape, so thefiber was touching the steel at only 2 points. The fibers were put in afurnace and annealed in air at 900° C. for 30 minutes.

Example 25

Films can be produced by dip coating onto a variety of substrates, steelbeing the most common. The sample is air dried, and then heated with aninfrared lamp to cure the coating. The coating cures much more quicklythan in the furnace, 30 sec to 3+ minutes depending on the substrate.This eliminates the step of putting the sample in the furnace andreduces substrate temperature and heating times. Species removed fromthe precursor state in order of their volatility are ethanol and otherhydrocarbons (below 100° C.), nitrates (typically above 500° C.), andhydroxyls (at least above 1000° C. in case of powders). For very thinfilms (below 500 angstroms), the temperature limits may be much lower.It is worth noting that the exothermic peak in the DTA around 225° C.suggests formation of the amorphous phosphate phase. Accordingly, apiece of stainless steel is half-dip coated in precursor solution. Thepiece is heated with the IR lamp for 2 minutes. The resulting pieceshows the bottom half well-coated and the top half still appears as itdid previously. In contrast, when stainless steel is half-dipped andannealed in the furnace, the bottom half shows a good compositionalcoating, but the whole piece is slightly discolored from oxidation.

Example 26

The compositions of this invention have been spin coated onto siliconand steel in a standard spin coater. Aluminum phosphate prepared asdescribed herein has also been coated in a 3-dimensional process ontosteel by immersing the piece and removing it, then spinning the wholepiece (e.g., use of a drill press). The coatings seem to be more uniformand have fewer cracks than standard dip coated pieces. Accordingly, apiece of stainless steel is immersed completely in a 6.6 g/L aluminumphosphate solution. The piece is withdrawn, and immediately spun (rpm tobe determined, but less than 540). The piece is cured with the IR lamp.The piece is cured slowly by slowly bringing the IR lamp closer to thepiece, over a period of 5 minutes.

Example 27

Because P₂O₅ is very hygroscopic, preparation is best carried out insidean dry glove box. To test the possibility of working in the openatmosphere, the P₂O₅ was weighed outside the glove box and left to sitovernight. Overall, the P₂O₅ picked up 3.8 g of water from an original19 g P₂O₅. This syrupy P₂O₅ was dissolved in ethanol, and an aluminumnitrate solution was added. The XRD pattern shows the desired aluminumphosphate composition after 1100° C. 1 h anneal, proving it can besynthesized in ambient atmospheres without using controlled environment,thus reducing the need for expensive atm control. Accordingly, 19.57 gP₂O₅ is weighed out and left to sit in the laboratory and was left for22 hours, taking up water to provide a syrupy consistency, instead ofpowder as it is when it is dry. This was dissolved in ethanol and addedto aluminum nitrate solution. The XRD shows the resulting aluminumphosphate after 1100° C., 1 hr anneal.

Example 28

Composition of this invention can be applied to glass by a dip coatingprocess. The resulting coating is very smooth and transparent. Where thecoating is continuous, it is featureless under the optical microscope,and is only noticeable when held up to the light. Coatings on glass areneeded for corrosion protection, as a glass-strengthening aid (healsurface flaws), and for altering optical properties. Accordingly, aglass microscope slide is dipped in a 17.6 g/L solution. The piece isblow dried with cold air until dry. A low power IR lamp provides gentleheat. After it is dry, the high power IR lamp is turned on and the pieceis heated for 4 minutes.

Example 29

Dip coated silicon can be annealed 1200° C., for extended periods oftime. Coatings on silicon may be useful in the semiconductor industry asa low dielectric stable coating (dielectric constants lower than 2.9 isdesired); a typical aluminum phosphate powder of this invention withdielectric constant as low as 3.3 has been made; further optimizationmay even lower it further to meet the 2.9 criteria, providing aninexpensive way to make these coatings and achieved such results.Accordingly, a piece of silicon was dip coated and annealed at 1200° C.for 180 hours. There is evidence of some coating degradation, as thereis no phosphorus evident in the TEM cross-section. Similar techniquescan be used to coat molybdenum substrates.

Example 30

Solution of the present invention can be spray dried. The resultantparticles had a mean diameter of 11.5 microns and were generally between5-25 microns. Powders annealed at 1100° C., 1 hr retained thecharacteristic spectral patterns.

Example 31

It has been determined that the Raman peaks at 1350 and 1600 cm⁻¹ arerelated to elemental carbon. It was also determined that the peak near1350 cm⁻¹ in some FTIR spectra showed was the result of atmosphericcontamination, not P═O. The presence of nanoinclusions of carbon isbelieved responsible for the black color of the powders of thisinvention. Nanocrystalline carbon (with a grain size as small as 15 Å)shows peaks at 1350 and 1600 cm⁻¹ in the Raman spectrum. Carbon has aweak IR spectrum, which explains why there are no carbon peaks in theFTIR.

XPS analysis was performed on annealed powders. Both as-annealedspecimens and crushed powders (to expose fresh surfaces) were analyzedby Physical Electronics Corporation (MN, USA). The as-annealed specimensshowed carbon content of less than 0.1% whereas the crushed powders didshow carbon presence near 1.6%. However, the report cast doubt on the1.6% for the crushed powder. The skepticism was based on powderdispersion in the chamber, and may be the result of some surfacecontamination not removed during extensive sputtering to remove 1500 Åof the surface from the crushed powder (500 Å was removed from theas-annealed powder to remove surface contamination that was reported tobe typical for any material exposed to air). This assessment was alsosupported by TEM and SEM analysis with low Z detectors, although thedetection limits with energy dispersive spectra (EDS) are generallyabove at least 1 wt %. In addition, no graphite inclusions have beenobserved within the amorphous matrix. It is indeed possible that thesize of these inclusions are below 5 nm and are randomly distributed orthat it is present in a glassy form mixed in with the amorphous oxidematrix.

Both Raman spectroscopy and CHNS (Carbon-Hydrogen-Nitrogen-Sulfur)analysis have confirmed the presence of carbon in the aluminum phosphatematerials of this invention. The amount of carbon present is indicatedby the color of the powder. Black powder contains more carbon thanlighter powder. Even powders described as “gray” or “light” are nottruly gray, they are a mixture of black and white domains that appeargray when crushed.

A pellet of the black composition/powder was sent to Oak Ridge NationalLab for testing in the “Kaiser rig.” The pellet was annealed at 1200° C.for 500 hours in a total pressure of 10 atmospheres, with 15% steam. Thepellet lost approximately 5 wt % during the experiment, but otherwiseappeared intact. The pellet was almost completely white. The surface wasremoved to eliminate effects of any surface contaminants and the pelletwas x-rayed. The X-ray diffraction pattern was similar to the originalpowder (FIG. 12). The XRD pattern did not indicate significantcrystallization. When observed under the optical microscope, the pellethad a few isolated black grains, but was over 98% white.

TEM analysis of the crushed pellet indicates that there arenanocrystalline inclusions embedded in an amorphous matrix (FIG. 13).Electron diffraction patterns show diffuse amorphous rings superimposedon spot patterns, which is typical for the compositions of thisinvention.

Raman spectra were taken. The microRaman used has a spatial resolutionof approximately 3-5 microns, so spectra could be taken of black andwhite domains in the same sample. Black areas consistently showed peaksnear 1350 and 1600 cm⁻¹. The intensity of these peaks scaled with eachother from sample to sample. White areas showed low intensity peakswhich lined up with crystalline berlinite, and did not show the peaks at1350 and 1600 cm⁻¹ (FIG. 14).

Further analysis is pending, but the results of this example areencouraging for use of this invention in steam environments includingits use as an environmental barrier coating in SiC-based composites usedin coal combustion applications where low oxygen diffusivity combinedwith resistance to high temperature steam are critical needs. Inaddition, use of such coatings for steam-laden atmospheres for mid-highT applications (such as petrochemical processing) are also relevant.

To illustrate the results of this example and just one application ofthe present invention, there is a need make coal-fired power generationmore energy efficient by increasing the combustion temperature. Greaterefficiency of power generation will help alleviate increasing demand forpower and reduce both solid and gas phase hazardous waste products. Asis demonstrated in the state of California recently, power demand isincreasing dramatically. California has been faced with rollingblackouts which have cost millions of dollars for businesses locatedthere. Certainly, more efficient power generation plants are necessaryto offset the increasing demand with limited environmental impact.

Currently, the temperatures in the boiler are 550-650° C. Commonly usedalloys do not have the required properties for use at 700° C. and above.The specifications required for next-generation Ultra-SupercriticalBoilers are high creep rupture strength at 750° C., and high corrosionresistance, with the loss of no more than 1 mm in cross-section after100,000 hours of service. Austenitic stainless steels are desirable as areplacement material because they are inexpensive and can maintainnecessary strength at high temperatures. However, these alloys encounterproblems both with high temperature oxidation and sulfidation andcorrosion by coal ash. There is also a problem with coal ash erosion ofsteel parts. However, the coal ash quickly coats the parts and, ineffect, forms a protective layer on the substrate.

Over the past decades, extensive R&D has been conducted on protection ofmetals and alloys in coal combustion environments. Many new alloys havebeen developed, along with coatings to slow the rate of degradation.Researchers have explored the corrosion resistance of commercialstainless steels, modified stainless steels, nickel-base alloys andothers. Ferritic stainless steels corrode by the formation of FeSO₄.They have found that nickel and cobalt containing alloys corrode easilybecause of the ease of formation of NiSO₄ and CoSO₄. Both of thesesulfates form a low-melting eutectic with Na₂SO₄, increasing thecorrosion of the alloy. High chromium content alloys (greater than 25%)show improved corrosion resistance, because a chromia scale grows fromoxidation. However, these alloys are subject to corrosion as well, whichis most severe at intermediate temperatures where the chromia growsslowly. The sulfur present in the combustion gas forms CrS₂ whichdegrades the quality of the oxide scale, further reducing itseffectiveness as a protective coating. In response to these problemsgroups have added other alloying elements to the steels, such astantalum and niobium which have increased corrosion resistance. Aluminumcontaining alloys and intermetallics (Fe₃Al) have been applied in hopeof developing an aluminum oxide scale which shows superior oxidation andcorrosion protection. However, these high-tech alloys and most coatingsare prohibitively expensive for widespread use.

A suitable system which attains the necessary specifications at areasonable cost for use for general power plant use has heretofore notbeen found. An ideal solution to this problem would be an inexpensivecoating that would serve as an oxidation and corrosion barrier for acommon austentic steel. The present invention provides an inexpensivematerial that can be applied very easily, and would be a low-costsolution to the problem. If the temperature of the plant can beincreased, the efficiency will also increase, leading to benefits ofincreased output power for a given amount of coal, which will givelower-cost energy, and environmental benefits of having to burn lesscoal. This not only saves money in processing power, but also reducesclean-up costs, which can be substantial. The compositions areinexpensive and easy to apply: spraying a solution onto the exterior ofa heat exchanger tube or inside a boiler, for example.

Example 32a

³¹P NMR spectra of a precursor solution for a preferred aluminumcomposition shows that the aluminum nitrate is interacting with thephosphorus pentoxide solution to form one or more unique complexes. Forbasis of comparison, the ³¹P NMR spectrum of phosphorus pentoxide inethanol is presented. FIG. 15 a shows the spectrum of P₂O₅ dissolved inethanol, and was taken shortly after dissolution. FIG. 15 b shows thesame solution after 24 hours of reflux.

Example 32b

The addition of aluminum nitrate Al(NO₃)₃.9H₂O to the phosphorusprecursor solution of Example 32a changes the ³¹P spectrumsignificantly. FIGS. 16 a-b show three spectra from three admixedprecursor solutions: the bottom curve is from C-1 (stoichiometric Al),and the other two are for increasing aluminum addition (C-1.5, 50%excess Al; and C-2, 100% excess Al). The differences between thesespectra and the spectra shown in FIG. 1 are readily apparent.

A set of new peaks appears between [−15 and −24 ppm]. None of thesepeaks are observed in the P₂O₅+ethanol precursor spectra. These peaksare believed due a complexation of the aluminum with the phosphorusspecies. A pattern is observed upon increase of Al content fromstoichiometric to a 2-fold excess.

Example 33

The solutions of Example 32b were annealed to high temperature (1100°C.) for long times (160 hrs.). The stoichiometric composition becomessomewhat crystalline over time, while those materials of this inventionwith excess aluminum provide XRD patterns showing substantial amorphouscharacter-denoting enhanced metastability. (See, FIGS. 17 a-b.)

Example 34

FTIR spectra of annealed materials/compositions of this invention showseveral unique features. For short anneal times (1 hr) bothstoichiometric (x=0) and non-stoichiometric (x=0.25, 0.5 and 0.75)compositions show similar features. The spectra are shown in FIG. 18.The spectra show predominately Al—O—P bonds, but show some featuresattributed to Al—O—Al and P—O—P. The Al—O—Al seems to be present inincreasing amounts with increasing aluminum content. Stoichiometriccompositions contain a very small amount of Al—O—Al. Stoichiometriccompositions show a fairly strong P—O—P feature, which is smaller inAl:P=1.25, and is very small (or nonexistent, it is hard to be certain)in Al:P=1.5 and does not show up in Al:P=1.75.

Example 35a

Stoichiometric compositions show distortion in the aluminum coordinationafter 1100° C., 1 hr anneal. FIG. 19 shows a deconvolution of thisspectrum. There are four curves in the deconvolution spectrum which addto the complete spectrum. The sharp peak near 39 ppm indicates Al inregular tetrahedral coordination. The other peaks indicate aluminum indistorted coordination and are listed in Table II.

TABLE II Deconvolution of ²⁷Al NMR spectrum of stoichiometric AlPO₄.tetrahedral peak octahedral peak position position relative area 38.163100 33.222 45.52 10.749 11.11 −16.356 14.84

Example 35b

Deconvolution of the ²⁷Al MAS NMR spectrum for a non-stoichiometriccomposition (x=1.0) shows there are distorted 4-fold aluminum species,along with more regular 4-fold aluminum (FIG. 20.) The peak near 40 ppmis tetrahedral aluminum, and the peak fit highlighted in green showsregular coordination, while the peak highlighted in red shows aluminumin distorted octahedral coordination. The regular 4-fold aluminum isbelieved present in the nanocrystals while the distorted 4-fold and6-fold aluminum is present in the amorphous matrix. Table III shows therelative peak positions and areas attributed to tetrahedral andoctahedral aluminum.

TABLE III octahedral peak position tetrahedral peak position relativearea −9.37 15.667 7.027 26.06 38.847 100 40.206 43.13 62.638 11.9

1. A method for protecting a substrate from corrosion and oxidation atelevated temperatures, said method comprising the steps of applying aprecursor solution to said substrate, said precursor solution comprisingphosphorus pentoxide and an aluminum salt, wherein the ratio of aluminumto phosphorus is greater than one to one, and thereafter dryingannealing said solution on said substrate.
 2. A metastable materialcomprising an aluminum phosphate composition having the formulaAl_(1+x)PO_(4+3x/2), wherein x is about 0 to about 1.5, said compositionhaving structural components absorbing radiation in the infra redspectrum at about 795 cm⁻¹ to about 850 cm⁻¹, said components present attemperatures of at least about 1000° C.
 3. The material of claim 2wherein x is about
 0. 4. The material of claim 3 wherein x is about 0.1to about 1.0.
 5. The material of claim 4 wherein said material issubstantially amorphous.
 6. The material of claim 4 wherein saidmaterial is metastable at temperatures at least about 1200° C.
 7. Thematerial of claim 4 further including crystalline particles.
 8. Thematerial of claim 7 wherein said crystalline particles are ErPO₄.
 9. Analuminum phosphate product having Al—O—Al structural moieties absorbingradiation in the infra red spectrum at about 795 cm⁻¹ to about 850 cm⁻¹,said product obtainable by mixing an alcoholic solution of phosphoruspentoxide with a solution of an aluminum salt, and heating theadmixture.
 10. The product of claim 9 wherein said product issubstantially amorphous.
 11. The product of claim 9 further includingcrystalline particles.
 12. The product of claim 11 wherein saidparticles are crystalline ErPO₄ inclusions prepared by incorporating anerbium salt with said aluminum salt solution.
 13. The product of claim 9further including metal oxide particles, said particles selected fromthe group consisting of Group IIIA and IIIB-VIB metal oxides, saidparticles in an amount sufficient to modify the thermal expansioncoefficient of said product.